This invention relates to printed matter, and especially to printed matter having imprints adapted to enable prevention or detection of fraud, i.e., counterfeits. In a more particular sense, the invention is directed to documents of value including new and improved intaglio imprints which resist counterfeiting, and to means and methods for producing such imprints.
Intaglio printing is widely employed for the production of documents of value such as bonds, bank notes, stock certificates and other papers as to which it is desired to prevent counterfeiting or enable detection of counterfeits. An intaglio imprint is obtained by pressing a suitable recording medium e.g., paper, against an engraved and inked plate so as to deform the medium into ink-bearing engraved recesses of the plate. The resultant impression, constituting a pattern formed of spaced and raised inked lines and/or other pattern elements, is characterized by superior clarity and sharpness of appearance, and cannot readily be copied with sufficient fidelity to escape expert detection.
Especially as produced with the aid of modern photographic copying techniques, however, counterfeits of intaglio-printed documents may be adequately authentic in appearance to escape detection upon cursory inspection by inexpert personnel; yet in many instances where detection of counterfeits is important, it is impracticable to subject documents to detailed or expert scrutiny. Therefore, in order to enhance the protection against counterfeits intended to be achieved by intaglio printing, it is desirable to provide means whereby relatively untrained persons can quickly and positively distinguish authentic intaglio-printed documents from photographic copies or like counterfeits, without the use of screens, special lights or other mechanical devices.
Some alternative expedients heretofore proposed for prevention or detection of counterfeits have involved provision, on a substrate such as paper, of a transitory image, i.e., an image that can be made to appear or disappear upon performance of certain operations.
Specifically, it has been proposed (e.g., Morris et al, U.S. Pat. No. 1,002,600) to provide a flat surface-printed pattern including an image which is formed of closely spaced parallel lines and is surrounded by a background of differently-oriented parallel lines of like shade so that the image is ordinarily difficult to distinguish from the background; for detection of the image, a screen bearing similar parallel lines is superimposed on the imprint in alignment with either the image lines or the background lines. It has also been proposed, e.g., in Dow U.S. Pat. 1,428,278, to print an image and surrounding background in different inks which are indistinguishable from each other in ordinary light but display a contrasting appearance when illuminated with light of particular colors. The production of transitory images by the use of lenticular screens, especially for purposes of amusement or artistic design, is well known, e.g., in Rice et al, U.S. Pat No. 3,241,429. Taylor, U.S. Pat. No. 1,990,421, proposed to impress an appropriately oriented pattern of embossed ridges on a substrate bearing a flat surface-printed pattern for production of decorative transitory variegated visual effects when the superimposed patterns are viewed at an angle at which the embossed ridges partially obscure the printed pattern.
Bayha, U.S. Pat No. 3,471,152 prints an intaglio pattern of parallel lines in ink having low contrast with the paper, e.g., white on white. He then detects the invisible pattern by an elaborate scanning apparatus (U.S. Pat. Nos. 3,457,421 and 3,462,225) which depend upon the absorption of electromagnetic radiation by his printed lines.
Expedients of the foregoing types, however, present either difficulties in fabrication, e.g., owing to problems of maintaining precise register during successive printing and forming steps, or difficulties in use, because they necessitate provision of auxiliary detecting devices such as special lights or screens. Even apart from these considerations, such expedients are not widely acceptable as substitutes for visible intaglio printing in security paper, for a variety of reasons including the marked superiority in quality of appearance afforded by intaglio printing, and the strongly established preference for intaglio printing in this field.